Signal Shaping Using Non-Unique Symbol Mappings
F. Schreckenbach, Patrick Henkel · 2005
We obtain a shaping gain with a non-uniform probability distribution of the signal points in a high order signal constellation. To obtain non-equiprobable signal points, we assign a non-uniform amount of distinct bit labels to the signal points. The proposed method is integrated in a bit interleaved coded modulation scheme with iterative demapping and decoding (so called BICM-ID scheme). This iterative process resolves the ambiguities that arise when several bit labels are assigned to one signal point. We derive a greedy algorithm to get a close to optimum probability distribution of the signal points and we use a binary switching algorithm to optimize the bit-mapping. At a rate of 3 bits per channel use, we obtain shaping gains over 1dB for a 16ASK signal constellation. In combination with irregular convolutional codes, we achieve an iterative decoding threshold at 0.7dB below the capacity of the uniform distribution. 1